This invention claims priority of the German patent application 100 46 144.1 which is incorporated by reference herein.
The invention refers to an arrangement for positioning substrates, in particular for positioning wafers, within a device that is provided for exposure of the substrates and/or for measurement on the substrates by means of radiation under high-vacuum conditions.
Arrangements of this kind are used principally in the semiconductor industry in chip production. In this context, with new lithography methods the need more and more often arises for novel and complex drive systems for particular utilization conditions such as high vacuum and environments with minimal magnetic or electrostatic interference fields. Positioning stages are used on which the substrates, in particular wafers, are placed. Displacement of the stages causes the substrates to be brought into the appropriate position for exposure or measurement.
As feature sizes in semiconductor technology become increasingly small, the physical limits of conventional photooptical lithography for the exposure of wafers are reached. The industry is therefore putting more and more effort into other methods such as, for example, ion beam lithography, electron beam lithography, or EUV (extreme ultraviolet) lithography. These methods usually place severe demands on the equipment, either because they are used under vacuum conditions or because of their sensitivity to electrostatic or magnetic interference fields. These demands apply in particular to a wafer stage for positioning the wafer during the ion beam lithography process.
The method for wafer exposure by means of an ion beam requires a wafer stage for positioning the wafer with respect to the exposing beam. The exposure process itself necessitates an ion source as well as a column of electrostatic lenses to focus the beam. Special wafer positioning requirements result therefrom. The sensitivity of the ion beam to electrostatic and magnetic interference requires the elimination of potential interference sources in the immediate vicinity of the exposure location, and careful shielding with respect to such sources (e.g. the wafer stage drives) elsewhere in the vicinity.
In addition, nonoptical exposure methods usually operate under high-vacuum conditions (in this case 10xe2x88x926 mbar). This results in problems with outgassing of materials and thermal stresses on power components. The drives that are used must therefore be selected with optimized power dissipation in mind, and if possible must be equipped with a cooling system. Materials that are not vacuum-compatible cannot be used. Friction and wear must be minimized or even eliminated.
The wafer stage must moreover make possible the exposure of wafers of different sizes, in particular 300 mm wafers, and for that purpose must possess a displacement range of at least 310xc3x97310 mm2. To compensate for wedge errors and focal position deviations on the wafer, wafer movements must be implemented in as many spatial degrees of freedom as possible. To assist the electronic beam tracking system (which operates in the nanometer range), an achievable positioning smoothness and accuracy in the sub-micrometer or microradian range is desirable. The measurement of optical reference marks on the wafer surface, which is required for coupling of the ion beam with the wafer, necessitates at least partially uniform displacement rates with a deviation of less than 2%. Not least, the wafer stage must make possible a high throughput of exposed wafers per unit time, and therefore must possess good dynamic properties.
The existing art does not offer any comparable systems that are designed for such different properties as operation in high vacuum, vertical working plane of the positioning system, compensation for the weight of the rotor arrangement using magnetic forces, and shielding of resulting magnetic fields down to a residual value in the nanotesla range, and that combine these features in one unit.
Conventional positioning systems for high precision usually meet the stated requirements for accuracy and dynamics by the fact that the moving elements are guided aerostatically. This does not, however, allow these systems to be used in vacuum.
Proceeding from this existing art, the object of the invention is that of further developing the positioning systems in such a way that greater positioning accuracy is achieved as a prerequisite for exact exposure and measurement of increasingly fine patterns even under high vacuum.
According to the present invention, what is provided for this purpose is an arrangement for positioning substrates of the kind described initially which comprises: a retention system, displaceable on a linear guidance system, for receiving the substrate, the guidance direction of the linear guidance system being oriented parallel or substantially parallel to the Y coordinate of an X, Y, Z spatial coordinate system; drives for modification of the inclination of the guidance direction relative to the Y coordinate; drives for rotation of the linear guidance system, including the retention system, about the guidance direction; and drives for parallel displacement of the linear guidance system, including the retention system, in the direction of the X coordinate, the Y coordinate, and/or the Z coordinate.
The invention is based on a novel six-axis positioning system having a zero-magnetic-field space, suitable for use in high vacuum for flat substrates, in particular wafers, in conjunction with exposure systems and measuring instruments using charged particles for irradiation, in which stringent requirements are applied for the elimination of interfering magnetic fields in the particle beam region. This positioning system is characterized by high precision and dynamics in all motion axes, and by great rigidity.
In a particularly preferred embodiment of the invention, two drive units are provided, each of which has a stator and a rotor with a modifiable air gap between them, the rotors being displaceable in the X direction and each rotor being joined to the opposite end of the linear guidance system for the retention system. A synchronous displacement of the two rotors causes a parallel displacement in the X direction, whereas an asynchronous displacement of the two rotors causes a change in inclination in the X direction (in other words, a rotation about the Z direction); and a synchronous change in the air gaps in the two linear motors causes a parallel displacement in the Z direction or a rotation about the Y direction, whereas an asynchronous change in the air gaps in the two linear motors causes a change in inclination in the Z direction (in other words, a rotation about the X direction).
It is thereby possible selectably to achieve parallel displacements in the direction of the X and/or Z coordinates, or rotations about the X, Y, and/or Z directions.
Each stator preferably contains, for use in vacuum, the drive coils of a linear motor acting in the X direction, and ferromagnetic guideways in the Z and Y directions. The rotor then carries the permanent magnet circuit of the linear motor, and electromagnetic actuators in which the ferromagnetic guideway of the stator is part of the respective magnetic circuit; as a result, the requisite bearing and drive forces between the rotor and the guideways of the stator are generated in noncontact fashion.
To control the air gap in the Z direction, each rotor is equipped with four such electromagnetic actuators that lie opposite each other in pairs at the respective stator; for each unit, the two actuator pairs have a spacing from one another measured in the X direction, and are activated so as to generate either an equilibrium of forces in a desired position, or requisite acceleration forces for positional changes. A noncontact magnetic guidance system in the Z direction, with an adjustable air gap, is thereby implemented.
At least a fifth such electromagnetic actuator is provided in order to generate the bearing force acting in the Y direction and as a drive for parallel displacement of the linear guidance system in the Y direction, activation of this actuator resulting in an influence on the air gap measured in the Y direction. A noncontact magnetic guidance system is thus implemented in the Y direction as well.
With this arrangement, it is advantageously possible to activate or control and reliably govern magnetic guidance systems or drives for six axes synchronously and in real time, thereby achieving changes in the position of the wafer surface in all six degrees of freedom, namely by displacements of the wafer in the X, Y, Z coordinate directions and by rotations about each of these coordinate directions, independently of one another in each case.
The retention system and the guideway of the linear guidance system are preferably made of nonmagnetic material. A stepping motor is present whose rotational motion is converted, via a Bowden cable system, into the linear motion of the retention system along the linear guidance system; and devices for clamping the retention system in a defined displacement position on the linear guidance system, which for example can be configured in the form of piezoactuators, are provided.
Advantageously, the retention system should substantially comprise a wafer chuck made of Zerodur for placement and electrostatic retention of the substrates to be exposed or measured, and a frame fabricated of titanium for mounting the wafer chuck. The linear guidance system should advantageously be fabricated of ceramic, the frame being coupled to the linear guidance system via lubrication-free ceramic ball bearings.
Interferometer arrangements that operate independently of one another are provided, for example, for measuring the respectively achieved displacement positions of the retention system and/or of the rotors in the X and Y directions.
In such a case mirror surfaces, which serve for measurement of the respectively achieved displacement positions using the interferometer arrangements that are provided, are machined onto the retention system, preferably onto the wafer chuck.
Three capacitative sensors that measure the distance of the substrate surface from a stipulated reference plane can be provided for measurement of the position of the substrate in the Z direction.
Also present in a particularly preferred embodiment of the invention are means for magnetic shielding of the regions in which the radiation used for exposure and/or measurement travels. This shielding protects said radiation from the influence of interfering magnetic fields, in particular from the magnetic fields of the drives for inclination change, rotation, and/or parallel displacement.
The shielding can be configured in the form of multi-layer shielding walls, the shielding walls, which are located between subassemblies that are displaceable with respect to one another, being offset laterally from one another so that meander-shaped magnetic seals are created.
The frame-mounted subassemblies of the linear motors, in particular coils, should be cooled. Provision can furthermore be made for equipping the movable subassemblies of the linear motors, in particular the rotors, with a thermally radiating surface coating.